PROCESS FOR ENRICHED COMBUSTION
USING SOLID ELECTROLYTE IONIC
CROSS REFERENCE TO RELATED
This is a continuation-in-part application of U.S. patent application Ser. No. 08/868,962, filed Jun. 5,1997, now U.S. Pat. No. 5,888,272.
FIELD OF THE INVENTION
The invention relates to the integration of oxygen enhanced combustion with oxygen separation processes that employ solid electrolyte ionic conductor membranes, and more particularly, to the integration of these processes to improve the economics, efficiency and pollution-related problems of combustion processes using a dilute oxygen stream as the oxidant.
BACKGROUND OF THE INVENTION
Many different oxygen separation systems, for example, organic polymer membrane systems, have been used to separate selected gases from air and other gas mixtures. Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. An entirely different type of membrane, however, can be made from certain inorganic oxides. These solid electrolyte membranes are made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides having a fluorite or perovskite structure.
Some of these solid oxides have the ability to conduct oxygen ions at elevated temperatures if an electric potential is applied across the membrane, that is, they are electricallydriven or ionic conductors only. Recent research has led to the development of solid oxides which have the ability to conduct oxygen ions at elevated temperatures if a chemical driving potential is applied. These pressure-driven ionic conductors or mixed conductors may be used as membranes for the extraction of oxygen from oxygen-containing gas streams if a sufficient partial oxygen pressure ratio is applied to provide the chemical driving potential. Since the selectivity of these materials for oxygen is infinite and oxygen fluxes generally several orders of magnitude higher than for conventional membranes can be obtained, attractive opportunities are created for the production of oxygen using these ion transport membranes.
Although the potential for these oxide ceramic materials as gas separation membranes is great, there are certain problems in their use. The most obvious difficulty is that all of the known oxide ceramic materials exhibit appreciable oxygen ion conductivity only at elevated temperatures. They usually must be operated well above 500° C, generally in the 600° C.-900° C. range. This limitation remains despite much research to find materials that work well at lower temperatures. Solid electrolyte ionic conductor technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is hereby incorporated by reference to more fully describe the state of the art.
Combustion processes, however, usually operate at high temperature and therefore there is the potential for efficiently integrating ion transport systems with oxygen enhanced combustion processes and the present invention involves
novel schemes for the integration of ion transport systems with oxygen enhanced combustion processes.
Most conventional combustion processes use the most convenient and abundant source of oxygen, that is, air. The
5 presence of nitrogen in air does not benefit the combustion process and, on the contrary, may create many problems. For example, nitrogen reacts with oxygen at combustion temperatures, forming nitrogen oxides (NOJ, an undesirable pollutant. In many instances, the products of combus
1° tion must be treated to reduce nitrogen oxide emissions below environmentally acceptable limits. Moreover, the presence of nitrogen increases the flue gas volume which in turn increases the heat losses in the flue gas and decreases the thermal efficiency of the combustion process. To miniis mize these problems, oxygen-enriched combustion (OEC) has been commercially practiced for many years. There are several benefits of oxygen-enriched combustion including reduced emissions (particularly nitrogen oxides), increased energy efficiency, reduced flue gas volume, cleaner and more
20 stable combustion, and the potential for increased thermodynamic efficiency in downstream cycles. These benefits of OEC, however, must be weighed against the cost of the oxygen that has to be manufactured for this application. As a consequence, the market for OEC is greatly dependent on
25 the cost of producing oxygen-enriched gas. It has been estimated that as much as 100,000 tons per day of oxygen would be required for the new markets in OEC if the cost of oxygen-enriched gas could be reduced to about $15/ton. It appears that gas separation processes employing ion trans
30 port membranes have the promise of reaching that goal. OEC is discussed in detail in H. Kobayashi, Oxygen Enriched Combustion System Performance Study, Vol. 1: Technical and Economic Analysis (Report #DOE/ID/ 12597), 1986, and Vol. 2: Market Assessment (Report #DOEI
35 ZD/12597-3), 1987, Union Carbide Company-Linde Division, Reports for the U.S. Dept. of Energy, Washington, D.C.).
Literature related to ion transport conductor technology for use in separating oxygen from a gas stream includes:
40 Hegarty, U.S. Pat. No. 4,545,787, entitled Process for Producing By-Product Oxygen from Turbine Power Generation, relates to a method of generating power from a compressed and heated air stream by removing oxygen from
45 the air stream, combusting a portion of the resultant air stream with a fuel stream, combining the combustion effluent with another portion of the resultant air stream, and expanding the final combustion product through a turbine to generate power. Hegarty mentions the use of silver compos
5Q ite membranes and composite metal oxide solid electrolyte membranes for removing oxygen from the air stream.
Kang et al., U.S. Pat. No. 5,516,359, entitled Integrated High Temperature Method for Oxygen Production, relates to a process for separating oxygen from heated and compressed
55 air using a solid electrolyte ionic conductor membrane where the nonpermeate product is heated further and passed through a turbine for power generation.
Mazanec et al, U.S. Pat. No. 5,160,713, entitled, Process for Separating Oxygen from an Oxygen-Containing Gas by
go Using a Bi-containing Mixed Metal Oxide Membrane, discloses bismuth-containing materials that can be used as oxygen ion conductors.
Publications related to oxygen-enriched or enhanced combustion (OEC) include the above-mentioned U.S. Dept.
65 of Energy reports authored by H. Kobayashi and H. Kobayashi, J. G. Boyle, J. G. Keller, J. B. Patton and R. C. Jain, Technical and Economic Evaluation of Oxygen